I had no idea people could get SQL Server - you know, that huge hosebeast of a database server - for free:
Granted it only supports 25 concurrent users, but seriously, how many people is 25 concurrent users? The way I figure it, if a single user is considered as connecting and disconnecting within 100ms after executing their query, that's a total of roughly 250 users per second or roughly 15,000 users per minute without collisions. Of course, that assumes perfect timing connecting and disconnecting at that rate, so I'll assume there is some delay and say roughly 7,000 users per minute to be safe.
Of course, if people are hanging onto their SQL connections longer than 100ms, then the number drops dramatically.
Basically, if you are using Access or Jet or whatever, you can use this as a drop-in replacement. Sort of an intermediate step to a full-blown SQL Server environment. At the very least, it will be faster executing queries and deal with any current concurrency issues.