How do I keep slime from collecting in my drip tray? My espresso machine has a pretty impressive drip tray, at just around 1.5 liters in capacity, it holds quite a bit. 

The problem is, if I empty it only when full, the bottom part of it has this really slippery, rather slimy surface which is extremely difficult to scrub. To be clear, it comes off easy enough, but tends to collect in nooks and corners.

I've tried adding a cap full of vinegar to the tray after cleaning and replacing it, this didn't make a noticeable difference. Even if I empty the tray when it's half full, there's still a bit of build up.

Is there anything else I can put in the bottom of the tray that wouldn't come with any competing odors to the coffee I'm brewing, but help keep this (as far as I can tell) 'water gunk' from forming? I'd really like to avoid dumping and cleaning the tray daily, as it is a bit of a chore.
If what you're seeing is mold or algae you may want to add a few table spoons of salt to the drip pan.

I'm guessing that your slime is probably a fungus of some kind, rather than an algae. It has found a rather nice living arrangement for a fungi. Plenty of warm water and decomposing coffee, with very little light.

Salt should in most cases prevent fungal growth, this may sound odd but it will dehydrate the cells even when they're immersed in water, sort of a "water, water everywhere but not a drop to drink".

If you would rather get high tech with it, you could try a UV light, similar to the ones used in AC systems. The harsh UV rays will kill most molds and prevent them from colonizing.