I've recently read an older blog on image resizing and I concider more has to be added to it.
All was pretty good but I’ve found a resizer at http://reshade.com that does all this and maintains the quality of the picture. There is no loss of edge sharpness and also I couldn’t notice any blur effects.

[Linked URL. -Philipp]

Throw a webcomic at reshade and the artifacts show up like there's no tomorrow. They have decent algorithms, but you're still faced with the fact that a smaller image has less data, and the larger has to extrapolate based on some algo or another – and that one algo will be better for some uses, while another will work best for other image types. Looks to me that Reshade is taking from the vectorization school of resizing.

Stanford born Vector Magic (http://www.vectormagic.com) tries to convert images into mathematical descriptions of curves, and then provides the vector files (as EPS or similar) that resize with no artifacting. It used to be free (http://www.bittbox.com/software/vector-magic-leaves-stanford-no-longer-free-desktop-coming-soon/)

Seam carving (http://www.faculty.idc.ac.il/arik/ and http://www.youtube.com/watch?v=vIFCV2spKtg) looks to preserve important information or extrapolate new information, but is computationally intensive – especially to handle both horizontal and vertical resizing.

The problem with Vector Magic is that now is a paid service. You get a few credits to start, and then you must pay.