I'm playing a Tiefling Gunslinger (3PP Mechanistic) into a Living Legends (3PP prestige class, Tome of Munitions) in a friends homebrew campaign, and my character is optimized in the same way that a brick is optimized to be aerodynamic. Really what I'm looking for is a way remain useful inside and outside combat.
I have a 16 INT so I can skill monkey it up a bit, and I get trapfinding from Mechanist so I'm the party's pseudo-rogue. In combat though my damage is pretty pathetic. I won't have Rapid Reload or Deadly aim until level 7, the remaining of which I'll have to take at level 9. The campaign is going to have few magic items in it, so I can't rely on that sort of thing. My Str is a 9 and I don't have the feats to get Weapon Finesse so melee is right out. What I'm really looking for is mundane or alchemical (and preferably cheap) ways of making my character just a little more than useless.
Any amazing / cheesy / amazingly cheesy mundane or alchemical item combo's you guys know of?
Ehm... you want RAW stuff or homebrew stuff?
Most of the interesting applications are rather situational and unplanned. Make sure you have a large inventory of items ('components') and be familiar with what you are carrying. By combining items you can do some incredibly useful stuff.
For RAW: smokesticks and smoke pellets remain useful until level 20 and beyond! Few things can see through mundane smoke.
I often use ground chalk in front of doors to see if they've been opened after my party passed them.
Caltrops are always useful.
For homebrew-ish stuff: I'm personally a fan of throwing pouches or empty empty eggshells of pepper, sneezing powder, finely ground glass, glue, oil, special herbs (e.g. wolfsbane or poison ivy) or a combination thereof. Especially lots of fun to throw in the faces of mages as a readied action when they are casting ;)
You can also make a bolas of a piece of rope and two flasks of Alchemist's Fire, Acid, glue etc...
You can make a wicked lasso/whip-type improvised weapon by dipping a piece of rope in glue and then in glass shards.
You can make simple noise traps with stuff like empty buckets, rope and/or doors. You can even fill the bucket with something like oil or grease.
If you can hold of a wand of Shrink Item you can do horrendous stuff. Imagine shrinking a tanglefoot bag and dropping it in someone's cup and saying the command word after they drank it.
Just try to find some lists of low-tech dirty tricks. There are plenty of articles on the web that tell of inventive but poorly equipped forces beating enemies that are superior in manpower and equipment (think of Vietnam).
[edit]: as I reread your post I realize you're probably looking for combat applications. Unfortunately, that'll probably be quite expensive if you do it with any regularity (20 gp for Alchemist's fire, ouch...). Probably easier and more fun to focus more on utility applications like the ones mentioned above.
"Stuff"
Good suggestions, I'll probably have to talk over the homebrew things with my GM, but he's the kind of guy who's open to cool stuff like this.
As for caltrops, I much prefer bear traps. They're bigger, meaner, and do 2d6+3. Sadly they weigh a ton and my carrying capacity is a pittance.