I voted for Frostburn simply because I think it has the most useful applications for a homebrew game. Of course...if you're homebrew centers on a city of the undead in the midst of a barren desert, then there are obviously better choices. So, it basically depends on what you want your next purchase to help you accomplish.
If it's a dwarf or gnome heavy game, Races of Stone is probably your best choice. If you want to expand your undead options, Libris Mortis was designed with you in mind! And if you want to do something with pirates and musketeers, then definitely check out AEG's Swashbuckling Adventures as someone else mentioned above. (I use this more than any other non-WotC sourcebook, though it does require significant tweaking for 3.5e use.) I would personally recommend you avoid the Monstrous Manual III unless you're in serious need of some bizarre, niche-filling monsters.