A common question that pops up soon after people have installed WSS/SPS (especially those familiar with 2001) is what about approval routing for content (particularly documents)? Well, the bad news is that there isn't really an "out of the box" approval "workflow" (like there was in 2001). However the good news is that there are a number of options, I have summarised a few of them below and I'm keen to hear if you have any others!

A) WSS provides a moderation facility (which is also accessible from SPS). While this is far from a true approval cycle, it may meet requirements for some scenarios. It's basically a one-step approval performed by any one of a group of administrators, until content is approved it will not appear. This features is available in both lists and document libraries.

B) WSS Document Libraries include an “Event” model. This allows customers and partners to attach their own custom code to events that occur inside document libraries (Save, Delete, Change, etc.). This is an advantage over SharePoint 2001 as it allows you to design a solution that meets your precise requirements. For further information take a look here:

Document Library Event Handler Toolkit: http://www.microsoft.com/downloads/details.aspx?FamilyId=4B2C2F1B-D74A-482A-903A-45BB44C5DEC4&displaylang=en

* Note this is only available on Document Libraries not Lists.

C) There are a number of third party solutions that provide this facility, I have listed below a couple that spring to mind, but there are sure to be others:

K2.NET Enterprise Workflow: http://www.k2workflow.com/solutions/collaboration/sharepoint.aspx
Ultimus: http://www.ultimus.com/index
Captaris Teamplate: http://www.teamplate.com/internal/product/sharepointworkflowwizard.asp
Nintext Smartlibrary: http://www.nintex.com/Product-SmartLibrary.aspx
LMR Solutions: http://www.lmrsolutions.com/products_workflow.asp
ADDED 28/7
Skelta Workflow.NET: http://www.skelta.com/ 

* Note that I do not personally recommend any of these products and the omission of any others is in no way a reflection on that products suitability or capabilities. It is up to individual organisations to decide if a third party solution meets their specific requirements.

D) Office has a basic client-based "approval cycle" built in, this is based on emailed versions of documents. Form the File menu, Send To -> Mail Recipient (for Review). No reason why you couldn't do some fancy client side coding (an Add-In/VBA) to extend this.

E) There are a number of community projects out there looking to provide workflow on SharePoint, here are just a couple:

Workflow Lite For SharePoint Beta Available:  http://weblogs.asp.net/jan/archive/2004/05/05/126614.aspx
Biztalk Server Adaptor for SharePoint Libraries: http://www.gotdotnet.com/Community/Workspaces/workspace.aspx?id=0d1aa85c-cf8d-497e-84f4-3ffec8db115f

F) The backward compatible document library is always an option. This is basically the document library functionality stripped out from SharePoint 2001 and shipped with [DMc EDIT - Removed WSS as this is only relevant to SPS] SPS 2003, this is the "Web Storage System" (Exchange-like store) based document store. This includes all the functionality that SharePoint 2001 provided "out of the box" and there is no reason why you couldn't continue to use this. Of course strategically, this is not the direction in which Microsoft is moving the product.