Pixelitor is an open source image editor that supports layers, layer masks, text layers, filters, multiple undo etc. and it has more than 110 image filters and color adjustments, some of which are unique to Pixelitor.
Note that Pixelitor is intended to be powerful rather than simple. You should have some experience with image editors in order to use it, because there isnt much documentation available yet, and many concepts (layers, blending modes, cropping, Gaussian blurring, unsharp masking, histograms, etc.) are the same.
* unlimited undo
* image filters
* layer masks
* text layers
This is it! All I need. Thanks.
Excellent program! Loads of features, all easy to use and implemented in a very practical and nice-looking GUI.