In huge fandoms, crack_van is invaluable if the reccer is on their game. Fandoms like Harry Potter and Sherlock or The Avengers can be awful places to try and find something decent to read, but the recs on LJ make it a lot easier. If something looks interesting but I don't have time to read it straight away, I'll use Read It Later (now Pocket?) to keep it where I'll find it again. Often I'll park stuff there for a couple of weeks then upload it all to my kindle at once.
If I could only choose one comm on LJ to join, it would have to be crack_van.