Besides the great pointers on the IntegratingSolr page, libraries integrating Solr have their own unique challenges that the rest of the computer programming world is advised to steer clear of.
How to index MaRC records into Solr
code4lib community maintained wiki of pointers and tips for working with MaRC with Solr
SolrMarc is a Java solution used by both Blacklight and VuFind, available at http://code.google.com/p/solrmarc/
Terry Reese has a video of how to use MarcEdit in this way: MARC -> Solr via MarcEdit
You can also use marc4j's command-line utilities to convert binary marc records to MARCXML, and use XSLT to convert these to solr add documents, but be warned that this process is significantly slower than other methods mentioned here.
Library Catalogs (OPACs) based on Solr
Blacklight: http://www.projectblacklight.org/ (Ruby on Rails)
fac-back-opac: http://code.google.com/p/fac-back-opac/ (Django/Python)
VuFind: http://www.vufind.org/ (PHP)
Other resources
There are many people on the code4lib mailing list or on the #code4lib irc channel who have tackled this issue and would be happy to help.