I am currently looking at two Solutions, the first one is a licensed set of two web parts - My Shared Documents & My Documents from http://www.salamandersoft.co.uk/ . I started with the My Shared Documents web part which allows users access to a shared folder on a file server. This is fairly easy to set up once you deploy the soluution as you edit an XML file with the UNC path of the file shares you want to make available and add it to the root of you SharePoint front ends. You can then drop the web part to any site. The web part also allows you to configure whether users can write back to the share or if it is read only.
The My Documents web part is even easier to configure as it picks up the users home directory settings from AD so there is no configuration of the XML file required. The My Shared Documents web part is shown below:
Notice that the context menu is very MOSS even though all the folders shown are on a file share on another server.
I also found a free web part from http://www.hezser.de/blog/archive/2008/03/30/access-fileserver-data-via-sharepoint.aspx that does the same thing but this one is security trimmed so you can point it to a share and it will only show folders and files you have access to. I have not had a chance to fully test this web part and will post an update once I have. I have found that the configuration options compared to the SalamanderSoft solution are very limited. Meanwhile, here is a screenshot:
As you can see this has a nice tree view.