Specially the older versions of PSP (7,8... while it still was made by Jasc) were great for pixel art editing.
Tons of filters available (with compatibility with even some from older photoshop versions), the grid, being able to work with custom palettes and defined number of colors and the option to see "brush outlines" make it perfect for pixel art!
Ranked in these QuestionsQuestion Ranking
Pro Custom palettes
Being able to work with a pre-defined number of color palettes, save and load custom palettes and several modes to reduce the number of colors on a picture.
Pro Several file formats supported
Can read and write images in several file formats: PCX, PNG, GIF, CDR, PSD, JPG, EPS, TIF, TGA, WMF... even RAW.
Pro Has tutotirals
There are quite a few tutorial videos available on Paint Shop's website for users.