I have used pretty much all of the products mentioned in the above posts. I would add the following to your sealer arsenal

1. Gorilla Snot (aka 3M Trim Adhesive). Use it like a heavy duty version of Gasgacinch
2. The red High Tack spray can be useful but kind of ugly looking.
3. On metal gaskets like BB valley tubs I use Aluminum spray paint that has aluminum particles in it. works as good as copper coat without the ugly color.
4. Some people swear by some of the OEM sealers like Yamabond and Hondabond.

Notes about RTV (silicone sealers).
1. Use it very sparingly. When squeezed thin, globs will squeeze inward and break off. Eventually getting caught in oil pickups.
2. It does work great in place of thick gaskets to seal gaps like where Intake manifolds that don't use tub gaskets sit above the china walls. Example: SBMopar
3. The extreme high temp versions can actually be used instead of header gaskets.