What free database software support sequences
If with "free" you mean "open source", then Firebird comes to mind (but I personally would prefer PostgreSQL over it - the much better documentation alone is a reason). 

Ingres also supports sequences, but I don't know if there is a .Net driver for it. 

There are other open source ones like Apache Derby, HSQLDB and H2 Database but as they are Java only, I'm pretty sure they do not have a .Net driver

If with "free" you mean "free of charge", then Oracle Express, DB2 Express-C and SQL Server 2012 Express are an option as well (as all three of them support sequences)