Log viewing utility database choice
My logs are very structured :)
I would say you don't need database you need search engine:
- Solr based on Lucene and it packages everything what you need together
- ElasticSearch another Lucene based search engine
- Sphinx nice thing is that you can use multiple sources per search index -- enrich your raw logs with other events
- Scribe Facebook way to search and collect logs
Update for @JustBob:Most of the mentioned solutions can work with flat file w/o affecting performance. All of then need inverted index which is the hardest part to build or maintain. You can update index in batch mode or on-line. Index can be stored in RDBMS, NoSQL, or custom "flat file" storage format (custom - maintained by search engine application)
You can find a lot of information here:
http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis
See which fits your needs.
Anyway for such a task NoSQL is the right choice.
You should also consider the learning curve, MongoDB / CouchDB, even though they don't perform such as Cassandra or Hadoop, they are easier to learn.
MongoDB being used by Craigslist to store old archives: http://www.10gen.com/presentations/mongodb-craigslist-one-year-later