I was searching for several resources for using JQuery in SharePoint. I was impressed by how well can we use JQuery to customize SharePoint.
Some very useful resources and demos you could find here (made by Paul Grenier):
http://www.endusersharepoint.com/?s=javascript
§ JQuery for Everyone: Replacing [Today]
§ JQuery for Everyone: Force Checkout
§ JQuery for Everyone: Degrading Dynamic Script Loader
§ JQuery for Everyone: Preview Pane for Calendar View
§ JQuery for Everyone: Preview Pane for Multiple Lists
§ JQuery for Everyone: Expand/Collapse All Groups
§ JQuery for Everyone: Fixing Configured Web Part Height
§ JQuery for Everyone: Total of Time Differences
§ JQuery for Everyone: Total Calculated Columns
§ JQuery for Everyone: Manually Resizing Web Parts
§ JQuery for Everyone: Dynamically Sizing Excel Web Parts
§ JQuery for Everyone: Fixing the Gantt View
§ JQuery for Everyone: Cleaning Windows Pt2
§ JQuery for Everyone: Cleaning Windows Pt1
§ JQuery for Everyone: Dressing-up Links Pt3
§ JQuery for Everyone: Dressing-up Links Pt2
§ JQuery for Everyone: Dressing-up Links Pt1
§ JQuery for Everyone: HTML Calculated Column
§ JQuery for Everyone: Print (Any) Web Part
§ JQuery for Everyone: Accordion Left Nav