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
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)