One thing to ensure if you are working with selecting resources that are compatible with RPG Maker is that the art is truly original and not simply derived from Enterbrain's packaged material, which has its own (understandably) restrictive license regarding use outside of RPG Maker. This is sometimes difficult to ascertain as even the artist may not always make it clear.
Finally, I'd like to thank the talented individuals who are willing to share their efforts with us less artistically gifted folk - they make many things possible that wouldn't otherwise be.
First Seed Material/REFMAP
First Seed Material/REFMAP (FSM/REFMAP) was a group that worked through a Japanese site called Tekepon, and had one of the most extensive RPG Maker compatible art sets not created by the makers of RPG Maker, Enterbrain. However, their site's hosting failed for one reason or the other, and an operation was put together to retrieve as many resources as possible. Look for iddalai's post for the best equivalent of a mirror.
The FSM materials are also highly significant because they are not derived from anything Enterbrain created and are capable of being used for free - even commercially - within the bounds of their license (see the license on that same forum - basically attribute by accreditation and linking to their site). Their corpus of art is a true gift to the community!
Also, if you hear about "Mack's tilesets" - that's part of this set. Mack was one of the members of the REFMAP group.
If you're looking for just one art set to get started with, start here. It is complete enough to make an entire RPG. I've also worked with trying to modernize the assets by finding the transparent colors and converting to PNG. If you're interested about these efforts, please comment.
An example of the FSM Materials - TileB from the VX assets:
Ayene-chan's DeviantArt Selection
Ayene-chan's art is excellent, for example see here. Her profile lists her terms of use which allow for commercial use as long as she is credited.
Sithjester's RMXP Resources
Sithjester's art is also exemplary, and contains some nice themed sets of art - free for the using as long as appropriate credit is given!
OpenGameArt.org
No list of links is complete without the up-and-coming OpenGameArt! This is a growing collection of art and artists sharing their work for you to use in your own game! However, please note the specific license applying to the work you are planning to use - it may not be available for commercial use.
One additional special thing about OpenGameArt is that they not only have visual art, but also music, sound effects, etc.
