TileWiz

An excellent tool! A few obvious feature suggestions:

• A view zoom, to enlarge low resolution tiles on high resolution screens and, less commonly, to shrink big tilesets or source images to fit them in a small window.
  Since only a few integer scaling factors would be needed, the zoom could be controlled by the mouse wheel or by a set of toggle buttons.
• Selecting and dragging rectangular selections as if they were big tiles.
  It can be useful both for faithful copying and swapping operations on groups of coherently arranged tiles (e.g. managing the 4 by 12 corner tile sets in the screenshot as 48 tiles) and for large scale editing (e.g. moving the rightmost  portion of the tileset a few columns away to make room for new tiles).
• Finding and selecting duplicate tiles, for example to delete them easily.
• An optional alternate tileset resizing mode: instead of leaving tiles at the previous locations and either adding empty rows and columns or obliterating tiles in deleted rows and columns, incrementing or decrementing the number of rows or columns could rearrange tiles (as if laid out in wrapped lines or columns) to fill the new size, and add columns or rows in the other direction if needed to fit all tiles.
  For example (. represents empty spaces)
  A B C
  D E .
  could be rearranged to 2 columns as
  A B
  C D
  E .
  rather than
  A B
  D E
  and to 3 rows as
  A E .
  D C .
  B . .
  rather than
  A B C
  D E .
  . . .
  There could be additional buttons ("reflow") in addition to the current + and - resizing buttons (which would be "extend" and "truncate").
• Specified offsets and margins around tiles; sometimes editing source images to align tiles is not a very practical option.