I like Mikroe MikroBASIC's demo, only limited in code size.
There is a (think it is) greatcows BASIC as well which is freeware.
Do not plan on MPLABS with a few of them. Unless you load the ASM and LST files to debug.
BoostBASIC way back hooked into MPLABS, not sure if they ever finished the project, I sort of grew tired waiting on them. If you go with the 18F chips, Swordfish is another good one to look at.