I use a multi-vitamin and a B12 supplement as I was deficient in B12. If you have deficiencies you will notice a difference, if your diet is poor then you probably are low in some vitamins.
Be careful with that combo as you might be over doing it on the B12 now. Probably a good idea to get blood tests again

(oh how I hate blood tests)
Too much B12 isn't much fun but if you're deficient maybe there's no problem.
I think supplements are great but as blonde said you gotta do a bit of research...swallowing a heap of random supplements isn't going to be much help.
Here's my experience with supplements/vitamins over the last 5ish years -
Had a blood test and they found my
Magnesium was a bit low. Have been taking 'chelated magnesium' at 500mg x 3 per day and saw some pretty good improvements with my energy levels and needing to sleep less. Got my levels up into the normal range and been taking it ever since.
Other magnesium bits - for people on amphetamines, either rec or medicinal, it can pay to have a bit of magnesium daily as it's supposed to help with tolerance. If you're on something like dexamphetamine for extended periods of time it might be a good idea as apparently these people when tested often have low magnesium (can't remember the cite)
The magnesium chelate is absorbed 'better' which is why I chose, sorry can't remember the source
Fish oil - some good findings on this over the last few years. I take blackmores 'omega brain' concentrated fish oil and find it helps with mental clarity, seems to slow down and steady my thoughts. Stopped taking this several times and after a while I notice the difference without it. Got a feeling it builds up in your body so doesn't matter if you miss a day.
The blackmores brand has been recommended to me by doctors and other people. Apparently the best quality and most potent one tablet a day. Should probably mention I was diagnosed with ADD as a kid but not sure if I still have it.
Went on a general
multi-vitamen for a few months but didn't notice any improvements so went back to my plain magnesium with fish oil.
Tyrosine - Awesome if you've been using stimulants then stop all of a sudden. Not usually does it have mood altering effects, but for people in stressful conditions with some sleep deprivation it's been shown to help. Makes me feel smooth with a certain slight 'buzz', even when I haven't had any stimulants in ages. I believe it's a precursor to dopamine?
Interested to read up on what other people have had luck with...only problem with this kind of thing is that it's so subjective/we're all different biologically/placebo can play a massive part.