Tile Layer is a video game ROM hacking utility program. It was written by SnowBro. It is a program which can read video game ROM files and interpret them so that 8*8 graphics tiles contained within the ROM can be viewed and edited.
Tile Layer can edit graphic tiles individually, or copy them onto a clipboard where they can be arranged easily. Also, if needed, two different ROM files can be loaded at once and graphics can be copied from one ROM file to another. Tile Layer can handle the following formats of file (which format the graphics are loaded in is interpreted from the file extension:
Two ROMs of any formats (same or different) can be loaded at once (from the DOS command line - the syntax is tlayer.exe <ROM file 1> <ROM file 2>) and then graphics from one can be placed on the clip board and copied into another. This is useful for hacking one game's graphics into another - for example, if you want to play as Link in Super Mario Brothers.
Tile Layer's other features include editing of individual tiles (either pixel by pixel, or using the Flip X / Flip Y / Rotate 900 commands) dynamic switching of graphics formats (in case SNES games store fonts in a lower BPP etc.), auto repairing of broken Game Boy checksums, offset Tuning (in case graphics are stored at, for example, $1B492 instead of $1B490 - offset tuning allows this to be taken into account and the graphics can be displayed correctly), customisable pallettes for each game you edit, and more. This really is an essential tool for any ROM hacker, the only excuse for not using it is if you use Tile Layer Pro.
yerricde adds that apart from this and Tile Layer Pro, another good Tile Editor to use is 8TED (http://www.pineight.com/pc/), especially because that one is open source. I hadn't heard of this before, but I will look into it. Thanks for pointing it out!