Can anyone suggest a best product/tool/site for growing twitter account for free ??

Tweetdeck is great to keep up with influencers, a certain topic or trending news if you're looking to get more involved with the Twitter community. :)

Try DeNet, 2 main functions Giveaway & LinkTool, both can gain followers and engagements for free. You can find it from website: https://denet.me/ and search from chrome web store by entering Denet.