One of the biggest troubles with playing real life (or, otb, as it seems to be so quaintly referred to on the internet) is that most people play between 20 and 30 games a year. Out of these, half should be black where you are subject to whites theory anyway, and the other half you will get to play your own stuff - provided that black does not come up with some wierd deviant move to take the game out of book.
Now, this means you will rarely get to employ any highly studied theory because 1.you will not often get a cooperative opponent who goes along with it long enough to get to a nice position for you and 2. it is very difficult to remember the precise moves from an opening which may well have been learned several years earlier and not yet had a chance to be used.
Having said this, I play, or at least have a system against the following openings.
I always open 1.e4 as white, apart from rare occasions in blitz where I have got sick of an opponents theory and want to mix it up a bit.
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4. Nxd4 (although, recently I have been experimenting with Qxd4)
if black plays ... Nf6 and a6, d6, e6 (all the same black system really, just different move orders) then I like to play into something like Nc3, Bc4 Qe2 etc... against the schvennigan and najdorf.
If black tries playing e6 early, or plays a kalasnikov, pelikan or accelerated dragon then I like to capture the knight on c6 and play g3 and Bg2 and keep the game fairly steady, picking at his centre pawns.
If black plays a dragon I like to play one of the more main lines of the yugoslav attack but will usually swap off dark squared bishops, allow the c3 exchange sacrifice and attempt to hold the rook until the endgame
Wierd type of moves tend to get met with g3 and a safe type of closed game to avoid any nonsense.
Alekhine: A tricky one, this, I have not yet found a perfect solution but I have been trying the chase variation which goes 1.e4 Nf6 2.e5 Nd5 3.c4 Nb6 4.c5 Nd5 5.Nc3
Also, the system of 1.e4 Nf6 2.e5 Nd5 3.Nc3 is ok for white and avoids any main line stuff which, you can be damn sure, black will know better then you do!
French: The exchange variation is a useful all purpose system 1.e4 e6 2.d4 d5 3.exd5 - playing a stronger opponent it offers good drawin chances and against lower graded opponents you get a chance at grinding them down while also frustrating them majorly as this is the kind of thing most french players REALLY hate to see - white not throwing material recklessly up the board trying to fluke a winning attack, while they gobble up everything offered and usually defend to safety - the exchange offers no major structural weaknesses or positional inbalance.
Caro Kahn: the panov botvinnik attack 1.e4 c6 2.d4 d5 3.exd5 cxd5 4.c4 is a nice aggressive system, and just the sort of thing caro kahners are trying to avoid - they can capture the c4 pawn and leave white with an isolated queens pawn, but if you keep the board crowded with minor pieces in these sort of positions, black tends to get himself in a horrible tangle as his pieces do not have anywhere sensible to develop to and you can develop good initiative with the space advantage.
If black does not capture on c4, eventually it is worth pushing it to c5 and trying to force your pawn majority through on the queenside - this is also quite a nasty cramped game for black to suffer.
Pirc - 150 attack is always fun and can yield good attacking positions where mates are possible, although the e4, d4 and c4 pawn wall system is quite handy too - you can keep the game closed and attempt to win on the queenside - beware of black counterattacks agaisnt your king though, they can be very fatal.
scandinavian- one of the hardest lines to meet as white as these buggers almost never play a consistent theoretical game and so you end up having to invent new moves every time. I have been experimenting lately with playing g3 and Bg2 and trying to clamp down on the queenside - I won my first attempt with it in a league game but it was a bit of a swindle really, so still needs time to perfect, but is nevertheless a setup which should be workable.
Also, the Tyrannosaurus gambit needs a mention here, 1.e4 d5 2.Nf3?! dxe5 3.Ng5 - a nice active game, where black is forced to open the board up, which is usually not in the style of such players so you are playing a psycological game here.
ruy lopez - the exchange variation is a nice line and very simple for white to play 1.e4 e5 2.Nf3 Nc6 3.Bb5 a5 4.Bxc6 dxc6 - whites entire plan now revolves around swapping all the pieces off, playing d4 to create himself a pawn majority on the kingside and then pushing them forward to win while black cannot free up his queenside pawns. This is such a long term plan that even the mighty engines cannot see it and is featured in one of my best wins against a banned user Game 4137275
the main lines are also good, and probably even better for white - I like the closed systems myself - but involve great deals of theory and are best not messed with unless you have really learned them well. I know quite alot of theory on my main system and a little bit on all the others, just enough to get me though, but I tend to avoid the critical lines in case my opponent knows more about it then me.
Against the petroff I play a relatively ordinary system, recently finding a great new innovation which I did not realise existed until watching a lecture by danny king.
1.e4 e5 2.Nf3 Nf6 3.Nxe5 e6 4.Nf3 Nxe4 5.Nc3 Nxc3 6.dxc3 Bg7 7.Qd4?! Bf6 8.Qf4 - the queen is now untouchable by any black forces trying to further molest it and can contribute heavily to any big attackqhite wishes to launch on the kingide.
Against the elephant gambit - take the pawn and run - you can hold it more or less all of the time as white.
Against the latvian gambit - dont try too hard to hold the pawn, unless black is not keen to recapture it. A move I have found to be reliable and strong against blacks setup is to play Bf4-g3 - this seriously stunts any black attempts to break open the kingside and leave you to get on with developing and breaking through on his queenside.
As black, I play the sicillian or e5 against e4 and d5 or g6 against d4 or c4 - anything else I just make it up on the spot and try to go with the principal of gaining space in the centre against flank systems.
There you are, I am sure there is plenty I missed out but at least it shows you what hard work playing 1.e4 is compared to 1.d4 and why it is the real mans opening!