There's various uprezzing software available: Genuine Fractals, Extensis Smartscale, S-Spline Pro (if I remember that name right).
I've run extensive tests with Genuine Fractals (you can download a trial version from http://www.download.com) and I compared it to Photoshop.... Genuine Fractals is supposed to be the best resizing software - I don't have any experience with the Extensis product.
If you need to resize an exisiting image to about 6x its scale, it arguably performs better than Photoshop. Below that, Photoshop does the business as good as GF!!! Just make sure you adjust the picture's tonal values ('contrast') before resizing: Photoshop takes the edge contrast into account whilst resizing: the higher the contrast, the 'sharper' the resized picture.
Don't expect any magic from any resizing software - a pixel is a pixel is a pixel.... not enough pixels for a requested ouput size results in an unsharp image. The resizing software is probably best used for big exterior adverts - I wouldn't reckommend it for any kind of magazine or book application. Also, the resizing software doesn't work that well with scanned images, due to the film grain.
I personally go for a Photoshop resize, followed by HighPass sharpening and/or other "sharpening" tricks like Photoshop's 'add grain' filter.
Hope that helps
Bjorn
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