This module solves the Internet Explorer limitation of loading not more than 31 CSS files per HTML page (http://support.microsoft.com/kb/262161).
The module will only kick in when it is needed, and remain silent otherwise.
This means, as long as Drupal's CSS aggregation is enabled, or your module count is very small, you will not notice any difference.
No configuration needed, just install / enable the module.
This problem is fixed in Drupal 7 core. Therefore, there will be no Drupal 7 version of this module.
IE CSS Optimizer: It does more or less the same as this module, with some subtle differences in implementation.
As an additional feature, it can aggregate all stylesheets except for those in specified themes or in the modules. Quite convenient, eh?
Advanced CSS/JS Aggregation (afaik) does everything that Unlimited CSS does, and a lot more.
It is definitely heavier, but do you care?
Differences to IE CSS Optimizer
IE CSS Optimizer uses @import instead of <link> only if there is really more than 31 stylesheets. Unlimited CSS has a lower "magic number" (22 max), considering that other modules or themes might add stylesheets in other ways than drupal_add_css().
If IE CSS Optimizer hits the magic number, it will use a combination of @import and <link>. Unlimited CSS will use all @import instead. Which method is preferable, would require further research.
Anyway, if we have custom stylesheets in the mix (which were not added with drupal_add_css), these will probably still be added with <link>, having us end up with a mix of @import and <link>.
As long as we are below the magic number (which is typically the case if CSS aggregation is disabled), both modules behave the same: All stylesheets will be embedded with <link>.
IE CSS Optimizer has options to optimize/aggregate all CSS files except for those in the module or theme being actively developed. Unlimited CSS does not have such options.
Both modules use hook_preprocess_page() to do their job, and are subject to the same potential conflicts with other modules.
If this module does not work as expected, it is very likely that you have another module or theme installed that overrides the stylesheet settings in hook_preprocess_page. You will find a copy of the modified $vars['styles'] in $vars['styles_unlimited_css'].