The acquisition possibilities for widget-like plays seems polarized based on their size of adoption. They seem to either get acquired very early on or once they have a massive following that impacts the usage of the 'host' ecosystem. Some examples:
MyBlogLog were aquired by Yahoo! very early on. They were a 'feature' rather than a service, but a feature none-the-less that Yahoo! needed. It was ultimately cheaper for Yahoo! to buy them out ($7m-$9m), gaining a proven proposition, rather than 'risk' building in-house. The early stage aquisition meant the valueation was low and cost effective.
The opposite I guess is Photobucket. Their gallery widget is a significant traffic driver and is used most prominently on MySpace. Recognizing that Photobucket had become so large that it was becoming a significant part of their service's usage Fox Interactive purchased the company for $300-350. Ultimately it was purchased because it was becoming an integral part of the proposition from the user's perspective and clearly the host provider (FIM) wanted to ensure it could fully control, owned and monetize it.
You mention that you are a 'moderate sized firm' and I wonder whether that will be a downfall to being acquired at this time. You maybe to large (and too great a valuation) to warrant purchase in a buy/build scenario by a large player. But too small that you don't have the size of following that makes it strategically important to acquire you for your userbase.
Nevertheless, to answer your question about "dealing with culture clashes and maintaining the usefulness of its products post-acquisition": I think it depends on many factors and the environment in which you are bought out. If you are considered a 'feature' rather than a 'product' then I would have thought you might be forced to roll over and do what your new overlords want. Chances are they will be buying you to fit into an existing larger strategy and you're simply a more cost effective way to acheiving that strategy then building in house.
However if you are a self-standing product with an established userbase then you have far more opportunity to maintain your own culture and doing things - after all that's what's attracted your userbase and that's what's given you the value you have bought to the company. A great example of this is Flickr, and Yahoo! have done well to stand back and let the property continue to run autonomously - often in ways that are very different from the rest of Yahoo Corp.
I think it therefore comes down to why the company has bought you and what their intentions are. Maybe it's worth fully exploring this before you sell to a possible suiters?