Yep, Lightzone is free for Linux, even though it's a commercial product for Windows. :-)
As for Picasa, Steve actually ran the story when Google made Picasa available for Linux (I sent him a note about it and it ran on our breaking news page here). It's got some quirks the way they're doing it. I haven't ran it under LInux in a while though (since one of the first betas).
Linux has some good tools available for editing images now, and most are free. Use the package management utility in your favorite Linux distro and search for UFRaw for starters.
Are you using Ubuntu (you mentioned that one)? Which version (6.06 LTS/Dapper) or 6.10 (Edgy)?
You can use Synaptic to install most stuff.
Also, get ufraw-gimp (it's a plugin for the Gimp).
This program is designed to work as a stand alone product or as a plugin for the Gimp (and make sure to download the Gimp, too).
UFRaw is doing the raw conversion and all edits in 16 bit mode, even when working as a Gimp plugin. It also works as a stand alone raw converter. The Gimp itself is just limited to 8 bits.
UFRaw is using the demosaic algorithms from dcraw.c, addding a nice GUI interface and more features. it's also available for Windows.
Another good product available for LInux that will convert your raw files is Eric Hyman's Bibble (also available for Windows). It has a pretty decent feature list available.
It's not free. But, you can download trial versions to see if you like it or not.
Also, make sure to check out DigiKam (free).
It's got a pretty good user interface, and I find myself preferring it's tools to some of the better known image editing applications for simple taks (USM, etc.). You'll also find it available in the repositories for most Linux distributions that use KDE. It's free and will convert your raw files, too.
If you're using Dapper (Ubuntu 6.06 LTS) digiKam 0.9.0 is not in the repositories yet (they're on the older 0.8.2 version which only supported 8 bit editing).
If you're running Ubuntu Dapper, I can give you links to download a couple of packages you'll need to install the newer 0.9.x version of digiKam in it (I'm running it under SimplyMEPIS 6.0 Final and SimplyMEPIS 6.5 Beta 6 right now and have the packages needed). These versions of SimplyMEPIS are using the Ubuntu Dapper respositories (and the new version isn't in the Dapper repositories yet).
Here are some screen shots (and make sure to install the kipi plugins for it).
I like it's highlight recovery better than Adobe Camera Raw. For example, the dog in this snapshot I took Monday was overexposed a good 2 stops according to the Exposure Slider in Adobe Camera Raw, and the JPEG was unrecoverable.
I liked this version better (using digiKam 0.9.0 setup to recover some of the highlights). I ran it through multiple converters, including ACR before I settled on digiKam's conversion.
It's a good thing I was in raw + jpeg and had the raw file. :-)
Another product to look at is Krita. KOffice 1.6 was just released, and a number of improvements were made to Krita (it's image editor), too. Krita now supports 16 bit editing, raw conversion, layers, color management and more.
As you already mentioined, another product worth looking at is Lightzone. It's user interface takes some getting used to. But, it's starting to grow on me a bit.
The Windows and Mac version of it are $149.95 or $249.95, depending on the version:
But, they offer it free for Linux users. BTW, I've seen users report that it's the fastest browser they've found for .raw files with Linux.
In a commercial (not free) product, Pixel supports color management with Linux and is an up and coming tool with 16 bit editing ability:
A lot of Linux users like Cinepaint for image editing. It's a GIMP "fork" (most code oritinally based on the Gimp). But, unlike the Gimp, it supports 16 bit editing.
A good way to do this is to use UFRaw for the raw conversion, color balancing, white point balancing, etc. Then, open the converted images using Cinepaint for any further tweaking desired (Unsharp Mask, etc.). It's free.
You can also get a number of Windows image editors to run under Linux via Wine (a free product that lets Windows apps run in Linux). . There are even dedicated sites to help you out with that part. Forget PS CS2 for now (although with a bit of tweaking, you can get earlier Photoshop versions to run OK with Linux under Wine).
I run products like Raw Shooter Essentials 2006 and the FastStone Image Viewer (both Windows products) in Linux via Wine. This site has more info on using Wine to run Windows apps (and many applications not in their lists run fine, too).
Make sure you install the msttcorefonts package, then install wine, then run winecfg to make sure fonts render OK (some Windows apps with have undereadable fonts if you don't have msttorefonts installed, since the fonts being used may not translate properly to the ones in the distro).
Another up and coming Graphics package that's free for Linux is Xara LX (recently renamed to Xara Extreme). It's pretty slick and powerful software and supports 16 bit edits (but, it doesn't have some of the tools that photographers may want yet like USM). For graphics designers, it would be hard to beat (and it's free).
There are many more out there. These are just some of them I've got installed on my PC.
I've got Windows XP Pro and more than one Linux distribution installed on my PC. Note that Windows XP is now at the bottom of my boot menu choices, since I don't use Windows very often anymore. lol