These are two new web-based applications I’ve found recently and just added to mind-mapping.org.
Web Sequence Diagrams
If you use UML Sequence Diagrams, this very easy to use browser-based application makes it really easy to make a diagram from plain text. You can download the diagram, or share a link to it, and even share a link to the page from which the diagram was made. There, others can modify your text to amend the diagram, so it supports collaborative development of sequence diagrams, though access would have to be sequential, not simultaneous.
So, I made an example:
Here’s the text that made that:
Customer->OrderTakingOnline:Order note right of OrderTakingOnline: adds prices, calculates invoice amount OrderTakingOnline->SalesDept:Order note right of SalesDept: verifies stock, Customer status SalesDept->FulfilmentDept:Order note right of SalesDept:Sends to Accounts SalesDept-->Billing:Order note right of FulfilmentDept:Requisitions Goods FulfilmentDept->Stores:Requisition note left of Stores:Issue Goods Stores->FulfilmentDept:Goods/Requisition note left of Billing: Raise Invoice/Packing List Billing-->FulfilmentDept:Invoice/Packing List FulfilmentDept->Customer:Goods/Invoice/Packing List note left of FulfilmentDept:Dispatch Shipment note right of Customer:(continues with payment, etc.)
You could even use it for a rather limited form of swim-lane diagram
This collaborative whiteboard application supports real-time, multi-user access not only allowing sharing and development of diagrams together, but also image up- and download, text chat and live audio.
Scribblar is free unless you want to have it on your own web site.