If you want to be notified of blockchain transactions that meet particular criteria and see how that affects prices, CryptoQuant is a great option. This on-chain and market data analytics service offers real-time alerts, along with a lot of on-chain and off-chain data, proprietary indicators and custom data analysis tools. It's geared for trading and risk management, so it's best suited for serious cryptocurrency investors.
If you want to see blockchain transactions in more detail and with more transparency, check out Arkham. This service specializes in deanonymizing blockchain transactions, which means it can help you figure out who's behind addresses, entities and assets. It offers custom alerts and a searchable database so you can monitor and visualize blockchain activity in detail.