Also ImmunityDebugger is pretty good: basically is OllyDbg with some tweak such as a built-in pythonAPI, a function graphing tool and a heap analysis tool.

Ida pro is really powerfull, but not as immediate and easy to use as OllyDbg; moreover, OllyDbg's plugins are actually useful.

Is now really outdated, but also W32DASM used to be good.

Finally, as stated above, if you want to get a bit more "in" the OS, there's Syser for your comfort. Syser is SoftIce's little up-to-date brother (as they say "Softice is left. Syser will continue.").