photokeywords is an experimental tool which allows you to gather keyword info from a microstock site. I have to say I'm not to sure I condone the harvesting of keywords like this, it's better than just cutting and pasting the keywords from another photo.. but not much!
The concept is simple, you enter a say three keywords and the site searches the database at shutterstock for some images. You select the images that are most like yours and then photokeywords displays all the keywords within those images as a list. It orders the listing by the most popularly searched keywords. You can then select and reject which of the keywords are relevant to your photo and finally a list of the keywords is displayed for you to cut and paste.
I was a little sceptical at first, and I did have some problems with firefox and the site which is clearly a developmental thing, they still list lucky oliver as one of the sites to search for keywords on! I probably won't add this to my normal workflow, but perhaps it's useful for the times when you just can't think of enough keywords for your images.
Have a look at stocktagger, I can imagine that this one is very useful if you need to translate your keywords from German to English. You enter a keywords (tags) and a list of related tags is displayed which you can add to your list or click on to perform another search based on that keyword or phrase, Instructions are on the site. You can then copy and paste comma separated keywords and phrases in either German or English. The service is free to use
Also see the comments below