I use the SharePoint List Filter web part on a page to control several Data View Web Parts.
SharePoint List Filter web part is rendered as input text field for displaying filter value:
and input hidden field for storing filter value:
Below are demonstrated several ways how to retrieve filter value on the client side.
Retrieve filter value from input hidden field (jQuery dependency)
Retrieve filter value from context info
And if you have multiple list filter webparts in your page, here is how to access the value of each of them
for your 1st webpart
for your second webpart, and so on
Another method I just found after more research:
vgrem provided a more beautiful version though ;)