non market android apps

So ive been poking around reading about unofficial android apps, or 'non market' apps. Im wondering how many of you use non market apps.

also im wondering if theres anything really GOOD out there thats non market.

for example, things i might want if i could find them that are non market:
email apps
music player
video player
sms apps

usefull type apps. Anyone have experience or input on the matter?

 

no body?