Solr Near Realtime Search
Near realtime search means thats documents are available for search almost immediately after being indexed - additions and updates to documents are seen in 'near' realtime.
You can now modify a commit command to be a 'soft' commit. A soft commit will avoid parts of the standard commit that can be costly. You still will want to do normal commits to ensure that documents are on stable storage, but soft commits allow users to see a very near realtime view of the index in the meantime. Be sure to pay special attention to cache and autowarm settings as they can have a significant impact on NRT performance.
You can read about soft commits here: http://wiki.apache.org/solr/UpdateXmlMessages#A.22commit.22_and_.22optimize.22
You can see how to auto soft commit here: http://wiki.apache.org/solr/SolrConfigXml?#Update_Handler_Section
A common configuration might be to 'hard' auto commit every 1-10 minutes and 'soft' auto commit every second. With this configuration, new documents will show up within about a second of being added, and if the power goes out, you will be certain to have a consistent index up to the last 'hard' commit.
You can also use commitWithin for fine grained control over NRT depending on the update. All commitWithin commits are now 'soft' commits by default.
There is also a blog post detailing some of the current improvements in this area on trunk located on SearchHub: http://searchhub.org/dev/2011/07/11/benchmarking-the-new-solr-near-realtime-improvements/