Much has been written on these… I will try and save you some pain by giving some best of breed links here along with what I believe to be the best methodology for quickly and efficiently solving this dilemma. If you want good background reading on URL re-direction in general go here.

You have several options for this with SharePoint 2007:

1. You could use an asp script as shown here (useful if you want to do wholesale redirection)
2. you could try one of several CodePlex Projects:  Smart 404 feature or RDA Collaboration (esp. SharePoint URL Redirector) (Note: I did not try either of these routes but they look interesting)
3. You could use the Host-Named Site Collection ability during site creation using the stsadm –o createsite –hhurl option (however many limitations exist that are somewhat hard to swallow in MOSS).  This is a good article on this subject
4. You could use one of Todd Klindt’s methods he posted here. However, he is using Windows Server 2003 in his example and he leaves out the DNS configuration issue…(Note: this is an excellent article on re-direction in SP with 2k3 server listing several options).  
5. The method that works the best is a variation of Todd Klindt’s methodology modified for Windows Server 2008.  This is an excellent article on this topic in general.

Some things to note:

lets say that you have an URL of http://Corporation.inc/Sites/HR and you want people to access this via just http://HR in the browser…

You would use Method 5 to set up the IIS 7 for this re-direction BUT in my experience I ALSO had to modify the security settings for this new Site:  I ENABLED Anonymous Authentication and Disabled ASP.NET Impersonation for this new site (Note: Anonymous Authentication here is not that big of a security issue as this location (http://HR is simply re-directing all users to the REAL location (i.e. http://Corporation.inc/Sites/HR at which point the Site’s Security will kick in and be enforced before any data is actually displayed).

After this you also need to configre DNS to support the use of this new name:

• Open DNSmgmt console on a domain controller and add a CNAME record for your Alias re-directing your users to the real site.
  for example say you have 1 or more FEW servers you would NOT want to point this record to the SERVER’S physical name but rather to the Web Server’s address (i.e. instead of using ServerName.Corporation.inc use the web address that SharePoint is being served out of: YourSPWeb.Corporation.inc as this will allow for additional FEW servers to be added without additional DNS configuration.
  (Note: the reason that this will work is that the YourSPWeb.Corporation.inc FQDN has an A record that points to an IP Address that is mapped to your Web Farm.)
  Now when you open your browser and type HR into the URL an press Enter you will be re-directed by DNS to the Web Server Farm which will look at the URL HR and using the HTTP Redirect ability return the actual URL of http://Corporation.inc/Sites/HR instead.

Hope this helps

Jim