That doesn't look like Balsamiq to me. I'm not sure what that is but here are some others. Most wireframing tools run on Linux since most are either online tools (usually requiring Flash) or are written with AIR (e.g. Balsamiq) or Java.
HotGloo
http://www.hotgloo.com/
http://getsatisfaction.com/hotgloo
$7/m for 1 project, 5 users; Flash
Support collaboration.
No semantic (XML) export.
Mockflow
http://www.mockflow.com/
"Design, collaborate (in real-time) user interface mockups for your software and websites."
AIR, but now developing for iPad so they will have two different products to dev and support?
No community support or contact info (just a form)
Sitemaps
Mockingbird
http://gomockingbird.com/
Mockingbird is newish and HTML5 based; $9/month for 2 projects with many users.
"create, link together, preview, and share mockups of your website or application."
No annotations, no XML export.
Lumzy
Lumzy is a free, online Mockup and Prototype creation tool
http://www.lumzy.com/
No support site, no company info
Flash
Quplo
http://quplo.com/
Create mockups using HTML; Very new.
ForeUI; advanced mockups for websites & desktop + flowcharts
http://www.foreui.com/
$99; java app;
WireframeSketcher
http://wireframesketcher.com/
Very much an Eclipse plug-in.
Denim is free but is "better for use with a tablet"
http://dub.washington.edu:2007/denim/
Pencil
http://pencil.evolus.vn/
Mozilla Based, open source, free.
Native XML format.