I'd just be careful with that kind of stuff, first and foremost your connection speeds depend on your contract with ISP. If you have set up subscription for 10mbps connection, no software is going to make it faster than.
In case your connection speed is not fast as it should, remote server you're connecting to is most likely overloaded and bandwidth gets limited so that there's still enough bandwidth to access the site but download speeds would be limited during peak hours. In these situations download managers attempt to make multiple connections so to get around limitations but all browsers are capable of doings this already, plus most servers are set up to counter this by limiting the number of allowed connections per IP.
My point being, download accelerators are usually full of spyware, adware and all that fun stuff.