I'm currently evaluating which architecture is better for my startup project: I'm thinkin about hosting my own Solr instance (i've already used it in a Drupal project), using an external hosted Solr solution, using another engine (i.e. i discovered basex.org - anyone has used it? my data will be 90% XML) or something else. Probably I'll have to add this to the list...

edit: I didnt consider IndexTank but it seems interesting, I have just to understand if it's a good solution if you dont need social inputs